A well-known example of an application of a device for purifying a fluid as mentioned is an application in a vacuum cleaner, in which case the fluid to be filtered is air. Currently, two types of vacuum cleaners are known in the art and sold to consumers. A first type of vacuum cleaners is provided with a dust bag, whereas a second type of vacuum cleaners is not, and makes use of cyclones to separate dust and dirt particles from air.
In a vacuum cleaner comprising a dust bag, two filters are used, namely the dust bag for a majority of the dust and dirt particles, and a filter such as a so-called HEPA filter (High Efficiency Particulate Air filter) for very small particles.
A cyclonic vacuum cleaner comprises four components for filtering particles from air taken in by the vacuum cleaner during operation, namely a cyclonic pre-separator, a pre-separating grid, a cyclonic fine dust separator, and a HEPA filter. The application of a number of components is necessary in view of the fact that cyclones can filter particles with a sufficiently high mass density in a limited range of diameters. In order to be able to filter a wider range of particles, the dedicated separators as mentioned are needed, wherein                the cyclonic pre-separator serves for separating relatively big particles with a high mass density from the air;        the pre-separating grid serves for filtering relatively big particles with a low mass density from the air;        the cyclonic fine dust separator serves for separating relatively small particles from the air; and        the HEPA filter serves for separating even smaller particles from the air than the cyclonic fine dust separator.        
The pre-separating grid is used in view of the fact that the cyclonic pre-separator is not capable of separating particles with a low mass density like fibers, plastic foam and confetti from the air. However, the application of a grid has a disadvantage, which resides in the fact that a grid will clog. Hence, air flow resistance will increase during usage, and a constant suction power cannot be guaranteed.
Because of the need of fast rotating air on the hand and slow moving air for disposing particles on the other hand, cyclones are space-consuming solutions for separating dust and dirt particles from quantities of air. In a pre-separating unit comprising the cyclonic pre-separator and the pre-separating grid, it is the cyclonic pre-separator that needs a rather large volume compared to the pre-separating grid.